Freeman Dyson said that only Orion is so close to jumping off hte drawing boards and across the solar system, with no new inventions needed, and held the promise to address the true scale of the solar system.Theodore Taylor was a bomb designer, and later staunch anti-nuclear weapons activist, but he said that it still held hope as a peaceful use of the very technology that enabled city-killing weapons. He said that if you open the Pandora's box that is nuclear energy, immediately all these horrible things come out and we must say "no, we mustn't do that", but if you dig deeper, down towards the bottom you find the real content, which is hope.

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Nuclear pulse propulsionIf some of the ideas here strike you as a little unlikely, this one will seem downright reckless. The notion here is to power your spacecraft by periodically throwing a nuclear bomb out of the back and setting it off.

Nuclear pulse propulsion was studied seriously by the US government's military technology agency DARPA, under the code name Project Orion. The aim was to come up with a design for rapid interplanetary travel.

The design DARPA came up with was huge even by today's standards, and was built to be a giant shock absorber, with heavy radiation shielding to protect the passengers.

It seemed workable, but there were concerns about fallout if it was launched in the atmosphere as planned. The project was eventually dropped in the 1960s when the first nuclear test bans came into force.

Despite these worries, the Orion design remains one that could be built using existing technology, and some researchers are still coming up with new approaches to nuclear pulse propulsionhttp://dx.doi.org/10.1016/j.actaastro.2009.05.020. Theoretically, a nuclear-bomb-powered ship could reach up to 10 per cent of the speed of light, allowing a journey to the nearest star in about 40 years.

Plausibility: perfectly possible, if a tad hazardous

I always point out that if it'll take you to the nearest star in (too long) 40 years, it'll take you to the moons of Saturn or Jupiter, or the Jovian trojans or the Belt in a few months, or weeks to Mars.

About fallout: they were well aware of this, and the later variants were designed to be lifted above the atmosphere by rockets, to be fired in space only. (as well, the ultimate is that later, when we're out in space and really need the tools to open the whole solar system, big space-built ships are the big hope. From Earth orbit to the inner Oort cloud in a few years or several months, a few months back to Saturn's orbit, and weeks back to the inner solar system.)One version, if we allow it to start up from a high hyperbolic loft, took a single Saturn-V (or Energisa) launch to put a ~500 ton ship, with down to 21 days to Mars for a stripped-down passnenger ship. Another such version could take a slower trajectory, and the start-up (space launch) mass is =~50%+ payload at Mars orbit.If we don't allow it even at =~84km altitude, Energia could take it up in ~100 ton sections, for in-space assembly, and it's only fired above the Van Allen belts. If anything, this makes it even more technically feasible, since one concern is what happens if a bomb doesn't go off (2 or 3 per second, for the big ground-launched ships). Rapid cycling of the engine is risky, and if it's relaxed by being used only in space, you could stretch it out to a pulse every 20 seconds.

I'm all for strengthening treaties and establishing international law against nuclear weapons, including in-space, but this use of the technology just asks for multi-national cooperation. And (as a World Federalist) I'll say the purpose of treaties is just so that they can be backed out of, or ammended or changed by the signatories. We already know we need more multi-national cooperation to do things like mine the asteroids or exploit/inhabit space.

The bussard ramjet. I like that one because in theory, you could never run out of fuel. It's been used in Science Fiction to move objects the size of Earth's orbits, and it all seems logical. If you've ever read Larry Niven's "Ringworld" series, you would know all about them! I think yes to that, as many things could in theory be manipulated through magnetic field such as free floating space gases.

_________________Humanity is a disease. It's currently contained to this solar system. But once it's released, the universe shall tremble with fear.

They way I understand it, an Alcubierre drive would shrink space in front of the space craft and expand it behind it, thus pushing the region of space containing the space craft forward.

If you want an analogy, imagine you are floating in the air in a balloon on a windless day, with an airspeed of zero. Now you take the air in front of your balloon and cool it down, shrinking it, while at the same time heating and thereby expanding the air behind you. The patch of air that you are floating in will then be pushed forward, away from the expanding air behind it and towards the wake left in front of it. Your balloon will go with it.

You are now moving with a positive ground speed, but still with zero airspeed (as any balloon must have, since it doesn't have any propulsion). In principle, you could go as fast (in terms of ground speed) as you wanted by just speeding up your patch of air, without breaking the rule that your air speed must be zero.

An Alcubierre drive does the same thing, except not with air but with space-time itself. It makes a space craft's patch of space move at more than the speed of light relative to other parts of space, while keeping the speed of the space craft relative to the patch of space it's in at zero (or below the speed of light, anyway).

There's one little problem here, and that is how to warp space-time in just the right (and a practical) way. I think it'll take Zephram a couple more centuries to figure that out, if he ever does...

_________________Say, can you feel the thunder in the air? Just like the moment ’fore it hits – then it’s everywhereWhat is this spell we’re under, do you care? The might to rise above it is now within your sphereMachinae Supremacy – Sid Icarus

Well, does gravity warp space? Because if it does, and we can use it to make an alcubbier drive to send, not matter, but light, then we could use it to communicate at faster-than-light speed, which would be pretty useful . . .

_________________“Once you have tasted flight, you will forever walk the earth with your eyes turned skyward, for there you have been, and there you will always long to return.” -Anonymous

Oh, and won't expanding hot air have a smaller density than the cold air, leading the balloon to be sucked in the direction of the heated air? This is how an airplane works, yes? For that matter, this is how a balloon, works, yes?

_________________“Once you have tasted flight, you will forever walk the earth with your eyes turned skyward, for there you have been, and there you will always long to return.” -Anonymous

A more accurate description of general relativity would, I think, be that mass warps space, and that that warped space is what we experience as gravity.

Newton's laws say that anything that has no force working on it will keep moving in a straight line at the same speed. Einstein adds that that is perfectly correct, but that what we think is a straight line actually isn't, because we're comparing the line to the grid (1) on the paper (2) that it's drawn on, and both line and grid are misshapen due to the water colours we used to paint some objects onto it (3).

And so it appears that when we drop something it accelerates downwards due to the force of gravity, but in fact there is no force acting on it at all (when falling you are weightless) and it is simply moving in a straight line at a constant speed through a deformed section of space-time.

(1) frame of reference, (2) space-time, (3) mass distorts space-time

As for my balloon, yes, the analogy is flawed. I'm not sure what would happen in reality, perhaps the expanding air would first push the balloon away, then escape upwards due to its buoyancy, and the balloon would be sucked back towards the newly created low pressure area. But it was a description of an Alcubierre drive, not a lesson in thermodynamics .

_________________Say, can you feel the thunder in the air? Just like the moment ’fore it hits – then it’s everywhereWhat is this spell we’re under, do you care? The might to rise above it is now within your sphereMachinae Supremacy – Sid Icarus

Well, does gravity warp space? Because if it does, and we can use it to make an alcubbier drive to send, not matter, but light, then we could use it to communicate at faster-than-light speed, which would be pretty useful . . .

I believe a lot of SF stories use as a premise that in order to communicate faster than light you need to use a FTL ship to do so rather than light itself (ie radio).

Yes, or if they need to speed things up for dramatic purposes, you invent a FTL "subspace" radio.

There actually might be a way of creating a FTL communications medium in the synchronous particle pairs (can't recall the technical term or a link offhand). Basically linked pairs of particles which spin in the same direction or charge regardless of how physically distant they are. So with just one pair, separated and sent somewhere else, you can have a digital link. If you also have a means or assembling things on an atomic level, you can use this as an estaz-FTL "transporter" by "faxing" the information to assemble your ship or people on the other side. Still just theoretical.

One way I've seen a "warp" or alcubbier drive postulated is to magnetically harness a singularity or black hole and use its gravity to distort space. The ship falls forward towards the black hole while it pushes it away with magnetic repulsion. Yes it would have to be a small black hole. And they aren't exactly common (hopefully).

Another practical problem with Orion is that fission bombs don't completely consume their fuel. A large part of it doesn't react but it and its casing gets blown apart and accelerated at high velocities. This will have the effect of "polluting" space with high energy radioactive particles to fall back to Earth or collide with other spacecraft.